Electric heaters



ELECTRIC HEATERS j; Filed Au 15. l93f' INVENTORS John F. Hill Dean L. Mayer Thom A. Hu hes ATTORNEY 'an electrical insulating material.

3,254,320 ELECTRIC HEATERS John F. Hill and Dean L. Mayer, Alliance, and Thomas Alvin Hughes, Dayton, Ohio, assignors to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey.

Filed Aug. 15, 1963, Ser. No. 302,233

2 Claims. (Cl. 338-241) This invention relates in general to electric heaters and more specifically to high capacity immersion heaters which are capable of operating with a high heat output per unit area of the heater when submerged in a high pressure,

vaporizable liquid.

Immersion heaters, as now known in the art, are limited by the comparatively low heat output obtainable per unit area of theheater, the specific conditions under which they may be operated, or a combination of these two factors. Usually when a heater is used to impart heat to a fluid which is at an elevated pressure, high heat output rates must be sacrificed ,to enable the heater to withstand the elevated pressure environment. Characteristically, heaters of the prior art are commonly formed with spaced wire or ribbon resistance elements within the heater sheath. These elements have less effective surface in the resistance element than that of the resistance element of the present invention. Further-more, many heaters of the prior art utilize an electrical insulating material which is soluble in water, e.g., magnesium oxide. This creates the undesirable hazard that the liquid which is being heated might become contaminated by the soluble insulating material should the covering sheath fail. In situa tions where the purity of the liquid being heated must be assured, as a precautionary measure, such heaters must be provided with a thicker covering sheath than would otherwise be necessary, thereby resulting in a reduction in the heat output per unit area of the heater and increasing its cost.

The present invention provides a high capacity immersion resistance heater that has a greatly increased heat output per unit area of the heater as compared to that obtainable with heaters of the prior art. The present heater has the largest possible resistance element heat emitting surface area for a cylindrical heater. Moreover, the heater is of substantially unitary construction so that high externally imposed pressures have little or no collapsing effect thereon. With this heater arrangement it is also possible to reduce the thickness of the covering sheath as compared to heaters of the prior art, for a given operating condition, advantageously resulting in higher heat .outputs. Moreover, the resistance element is separated from the covering sheath by only a thin film of The center of the heater element is occupied by a solid metallic rod which acts as an electrical conductor element for supplying electrical current to one end of the resistance element while at the same time providing a solid central core structurally adapted to resist externally applied fluid pressure. This heater design also has the desirable features of being relatively inexpensive and readily fabricated.

Accordingly, the present invention provides an electrical heater with a tubular electrical resistance element, a tubular covering sheath enclosing the resistance element 2 l with a thin, dense film of dielectric material disposed therebetween, and an electricalconductor element connected to one end of the tubular resistance element and extending therethrough to the opposite end of the resistance element and separated from the resistance element by an electrical insulating material.

The method of fabricating this electrical heater comprises the steps of centering a central conductor element within the resistance element and filling the space therebetween with a granulated electrical insulating material, swaging the assembly, inserting the swaged assembly into the covering sheath, filling the space therebetween with granulated electrical insulating material, and swaging the final assembly to compact the insulating material.

The various features of novelty which characterize the I invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1 is a longitudinal view of the heater of the present invention partly in section; and

FIG. 2 is an enlarged transverse section of the present invention taken along lines 2-2 of FIG. 1.

A longitudinal section of the high capacity immersion heater 10 is shown in FIG. 1. The heater 10 is comprised of a tubular electrical resistance element 12, a tubular covering sheath 14 and a central electrical conductor element 16, all coaxially arranged. Disposed in the annular space between the central conductor element 16 and the tubular resistance element 12 is a layer of electrical resistance-heating material such as Nichrome metal. nickel, 20% chromium), stainless steel, an ironnickel alloy, at copper-nickel alloy, or other such alloys as are well known in the art. The resistance element 12 is connected at one end, as by welding, to a plug-22 which is also joined to the central conductor element or rod 16, the latter being formed of a low electrical resistance material. The central conductor element extends coaxially through the heater element to the opposite end thereof where it is attached to a source of electrical current, not shown. The correspondingend of the resistance element 12 is also connected to the electrical current source so that a circuit is formed by the series connection of the central conductor rod and the tubular resistance element. Due to the fact that the central conductor rod has a relatively large cross-sectional area of metal as compared to the tubular resistance element, very little heat is generated therein. Thus, current flowing through the conductorrod and the resistance element concentrates substantially all ing steps. First, the central conductor rod 16 is dis- Patented May 31, 1966 ing material, one end of the annulus being plugged. This granulated insulating material is uniformly introduced into and packed within the annular space 18 either by continued vibration of the assembly during the filling opera tion, or by compacting small amounts of the material periodically as the annular space is filled, After the annular space 18 has been filled, the conductor rod and resistance element assembly is swaged, thereby reducing the outer diameter of the resistance element 12, while at the same time elongating the assembly. The swaging process also compacts the insulating material, resulting in a thin but dense electrical insulator between the central conductor rod and the tubular resistance element. The plug 22, connecting the tubular resistance element and the central conductor rod, is then joined to the central conductor rod and to the resistance element. The swaged assembly is then disposed coaxially within the tubular covering sheath 14 and the annular spacetherebetween is similarly filled with additional granulated electrical insulating material which may or may not be the same as that in space 18. The covering sheath then is swaged,.reducing the outer diameter and compacting the insulating material 20 to form a thin, dense film of dielectric material between the resistance element and the covering sheath. An end plug 24 is then integrally connected to the end of the covering sheath to form a fluid and pressure tight end for the heater assembly. A suitable closure, not shown, may then be attached to the opposite end of the heater assembly providing electrical terminals for the resistance element 12 and the central conductor rod 16.

Throughout the assembly procedure, tests can be made on the partially finished heater for continuity, shorts, wall thickness and Weld integrity. In order to obtain the desired heater output, it is possible, during assembly, to reduce the wall thickness of the resistance element by electro-polishing or other means.

Alternatively, the heater may be fabricated by assembling the resistance element 12, the central conductor rod 16 and the outer covering sheath l4 coaxially one with the other, plugging the annular spaces, filling both annular spaces with a granulated insulating material and then swaging the entire assembly to obtain the final heater element 10.

In fabricating the heater element of the present invention by either method there will be a thin layer of electrical insulating material between the resistance element and the covering sheath, and between the resistance element and the conducting element which is of uniform density and capable of withstanding high generated operating temperatures, and the resultant differential temperatures associated therewith. Advantageously, the

thickness of the electrical insulating material is made as thin as practical, consistent with the material being used,

. thereby minimizing the thermal gradient thereacross while at the same time providing the requisite electrical insulation. It should be noted that it is possible for the terminal end of the heater to have a larger diameter than the remainder of the heater element as illustrated in the upper portion of FIG. 1. This may be accomplished either by not swaging this portion of the heater, by swaging this portion to a lesser degree than the major portion of the heater, or by starting with a piece machined to a larger diameter at one end.

A specific example of a finished heater assembly as described above is one in which the cylindrical resistance element is fabricated of type 304 stainless steel having a wall thickness of 0.01 inch and formed into a cylinder 0.34 inch in diameter and 6 inches long. The resistance element is joined at one end by a stainless steel plug to a central conductor rod, also formed of type 304 stainless steel with a diameter of 0.25 inch. The covering sheath is a tubular element having a thickness of 0.03 inch and an outer diameter of 0.42 inch. The two annular spaces provided by the assembly are filled with powdered alumina and have, when finally assembled, a thickness between the central conductor rod and the resistance element of 0.020 inch, and 0.01 inch between the resistance element and the covering sheath. In operation with a cur rent of 500 amps at 13 volts, this heater produced ap proximately 820 watts per square inch throughout the 6 inch heater length.

The present invention thus provides a highcapacity immersion heater which is comparatively easy to fabricate. The heater of the present invention is an improvement over electrical heaters of the prior art because of several factors. First, the tubular resistance element issubstantially the same size as the heater since it is only slightly smaller in diameter than the sheath. Second, the present heater construction uses a thin and dense layer of a dielectric or insulating material to prevent short circuiting between the resistance element and the covering sheath and between the heater element and the central conductor rod. The fact that the layer between the resistance element and the covering sheath is very thin, in the example cited above"0.01 inch thick, reduces the thermal resistance between the resistance element and the sheath, thereby promoting the rapid transfer of heat to the sheath surface. Third, swaging of the heater assembly insures that each portion or component is in intimate contact with the other portions, namely the resistance element with the layer of dielectric material, and this layer of dielectrical material with the surface of the covering sheath which results in the elimination of air spaces, and other thermal insulators, which would act to reduce the output of the heater and increase the center rod temperature decreasing the structural rigidity of the heater.

With the immersion heater herein described, we have been able to achieve a high capacity output per unit area of the heater with a simple, rugged design. As a cornparison, the presently available heaters have a heat output of approximately watts per square inch of heater outside area. As noted above, the heater herein described has achieved output rates of over 800 watts per square inch of heater outside area.

While in accordance with the provisions of the statutes,

there is illustrated and described herein a specific embodiment of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

What is claimed is:

1. An electric heater comprising a substantially imperforate tubular electric resistance element, a tubular covering sheath coaxially enclosing said tubular resistance element, a thin dense and uniform film of dielectric material disposed between said resistance element and said covering sheath, a low resistance conductor rod' disposed coaxially within said resistance element and having one end arranged at one end of said resistance ele ment, said conductor rod extending through said resistance element to the opposite end thereof, a connecting member having an outer dimension less than the inner diameter of said covering sheath connecting said one ends of said conductor rod and said resistance element, an end plug arranged to' be axially spacedfrom said connecting member and sealingly closing the end of said covering sheath, means connecting the ends of said conductor rod and said resistance element opposite said connected end to a source of electric power, and an electric insulating material disposed between said conductor rod and said resistance element.

2. A swaged electric heater comprising a swaged substantially imperforate tubular electric resistance element, a swaged tubular covering sheath coaxially enclosing said swaged tubular resistance element, a thin dense and uniform swaged film of dielectric material disposed between said swaged resistance element and said covering sheath, a low resistance conductor rod disposed coaxially within said swaged resistance element and having one end arranged at one end of said swaged resistance element, said conductor rod extending through said swaged re'sistance element to the opposite end thereof, a connecting member having an outer dimension less than the inner diameter of said swaged covering sheath connecting said one ends of said conductor rod and said swaged resistance element, an end plug arranged to be axially spaced from said connecting member and sealingly closing the end of said swaged covering sheath, means connecting the ends of said conductor rod and said sWaged resistance element opposite said c onnected end to a source of electric power, and a swaged electric insulating material disposed between said conductor rod and said swaged resistance element.

References Cited by the Examiner UNITED STATES PATENTS RICHARD M. wooo, Primal Examiner.

V. Y. MAYEWSKY, Assistant Examiner. 

1. AN ELECTRIC HEATER COMPRISING A SUBSTANTIALLY IMPERFORATE TUBULAR ELECTRIC RESISTANCE ELEMENT, A TUBULAR COVERING SHEATH COAXIALLY ENCLOSING SAID TUBULAR RESISTANCE ELEMENT, A THIN DENSE AND UNIFORM FILM OF DIELECTRIC MATERIAL DISPOSED BETWEEN SAID RESISTANCE ELEMENT AND SAID COVERING SHEATH, A LOW RESISTANCE CONDUCTOR ROD DISPOSED COAXIALLY WITHIN SAID RESISTANCE ELEMENT AND HAVING ONE END ARRANGED AT ONE END OF SAID RESISTANCE ELEMENT, SAID CONDUCTOR ROD EXTENDING THROUGH SAID RESISTANCE ELEMENT TO THE OPPOSITE END THEREOF, A CONNECTING MEMBER HAVING AN OUTER DIMENSION LESS THAN THE INNER DIAMETER OF SAID COVERING SHEATH CONNECTING SAID ONE ENDS OF SAID CONDUCTOR ROD AND SAID RESISTANCE ELEMENT, AN END PLUG ARRANGED TO BE AXIALLY SPACED FROM SAID CONNECTING MEMBER AND SEALINGLY CLOSING THE END OF SAID COVERING SHEATH, MEANS CONNECTING THE ENDS OF SAID CONDUCTOR ROD AND SAID RESISTANCE ELEMENT OPPOSITE SAID CONNECTED END TO A SOURCE OF ELECTRIC POWER, AND AN ELECTRIC INSULATING MATERIAL DISPOSED BETWEEN SAID CONDUCTOR ROD AND SAID RESISTANCE ELEMENT. 